I figure the one just sat around for longer, as I've had extra blue-y cheese develop in my own fridge. Just never seen such a striking difference at the store, which I found mildly interesting.
Softer cheeses often have moisture in/on them. Even some older "dry" cheeses can have a lot of moisture on the outside. I just had an 18-year cheddar this weekend and it looked like a slimy petri dish on the outside, but goddam that shit was fantastic.